Although much progress has been made in determining the mechanisms of regulation of cyclin/cyclin dependent kinase activation, little is known about substrates for these protein kinase complexes. Still, it is considered likely that cyclins contribute to substrate selectivity for the different complexes, resulting in the specificity of e.g. Cln2/Cdc28 for Gl/S events vs. that of Clb2/Cdc28 for G2/M events (note that the Cdc28 protein kinase catalytic subunit is the same in both cases). In order to examine this idea critically, and in order to begin to elucidate the mechanisms of action of cyclin-dependent kinases in triggering diverse cell cycle transitions, it is necessary to identify and characterize phosphorylation substrates of these complexes, to determine if any or all are only specifically phosphorylated by individual cyclin/Cdc28 complexes, and to determine the functional consequences of these phosphorylation events for cell cycle control. We are designing experiments that will utilize our newly developed MS approaches to assist in the identification of these potentially important phosphorylation substrates.